How to Repair Cracks in Stucco Walls


Cracks appear in stucco walls of buildings because the ground actually moves, and the building actually moves, and with time and the wet/dry/wet/dry annual seasonal patterns, the ground swells-and-shrinks and the building settles somewhat unpredictably over the decades.  The stucco walls don’t stretch, not worth-a-hoot.  When the building moves, then one part of the foundation goes a bit up, down or sideways relative to another part, and then the stucco cracks.

Stucco is basically cement.  When painted, it is very weather-resistant and does not have the rot-issues of wood; that’s why it is used on so many buildings.

Repairing a crack in stucco is often done with various caulks or cement, and usually the crack opens up again.  There are two reasons:  The cement patch can’t stretch, and the caulk did not stick.  These have underlying them one common denominator:  the building is still moving.  Patching the crack did nothing to stabilize the foundation, or change the seasonal swelling-and-shrinkage of the ground.  The crack is going to continue to open-and-close, and usually widens, on-the-average, as time goes by.

Furthermore, the crack movement usually exceeds the elastic limits of whatever-was-used to fill the crack, even if it was not cement.  Here’s why:

Let’s say that we have a reasonably stable building, and the settling factor happened over many years.  However, the ground swelling-and-shrinkage happens seasonally.  So, eventually a crack starts, and over many years it slowly widens.  But, from winter-to-summer-to-winter-to-summer, it is also opening-and-closing-and-opening-and-closing, due to the seasonal ground movement.  So, the crack might be 1/16” one year, and 1/8” five years later.  But, from winter-to-summer-to-winter-to-summer, it might open-and-close by 1/16” more-or-less.  Early in the cycle we have a crack that goes from zero to 1/8”.  Later in the cycle it might go from 1/16” to 3/16”, and then 1/8”-to-1/4”, and so-on.  There just is not anything that will stretch that much and still stay stuck to the stucco, which has not got great tensile strength anyway.  If you even used an epoxy glue in the crack, it would just pull off some stucco when the building asked the crack to open-up more.

This is a Mechanical problem, and it needs a Mechanical solution.  The simplest and least expensive one I know of is to vee-groove the crack, so it is much wider at the surface, such as a half-inch for a 1/16” crack.  Now, when that crack opens-and-closes by 1/16” more-or-less, the surface stretches-or-shrinks by a sixteenth out of ½, which is one out-of-eight, and that’s a very mild elongation; one that even a low-quality (inexpensive) latex-acrylic caulk can tolerate.  However, if you want a really long-lasting repair, I’d recommend something that does not shrink when curing and sticks by real chemical bonding, and that’s a sealant called 5200, made by 3M.

The idea is not to fill the entire crack or even the entire vee with the caulk, but to fill the upper part of the vee at least.  A piece of twine or soft rope can be pressed into the bottom of the vee, so the sealant does not go into the crack itself.  Actually, you don’t want it in the crack itself; if it did, then when the building tried to push that crack closed, it would just act as a wedge and another crack in the stucco would eventually open up somewhere else. Incidentally, that’s another reason why filling cracks with cement does not work in the long term.

Stucco is similar to wood in that it does not have a strongly attached surface; old stucco is often somewhat crumbly, due at least in part to its having been made with a lot of air mixed in with the cement-and-sand-and-gravel.  Some stucco is even sprayed, and that can easily be aerated.  Whipping-in air minimizes shrinkage-and-cracking while curing, and also air costs less than stucco.  What this means is that we need some kind of primer to glue and strengthen the stucco surface, and also something to chemically bond to whatever-kind-of-caulk you are going to use.  Smith & Company makes a really good one, and it’s called Permanent Stucco Primer.

That’s enough Theory.  Now, let’s talk about Products, and getting the job done.

The cement surface must be reasonably dry before continuing.

Moving air is the most efficient means of evaporating water. 

Now, we are ready to begin product application.  There are two products needed to perform the repair, a liquid and the gel-caulk. The liquid is the Smith & Co.  Damp Concrete Primer (DCP).  .  It won’t take much…an ounce or so of the mix is enough to prime at least ten feet of typical crack.  It is prepared by mixing one part of a liquid-concentrate with two parts of water.  A tan emulsion will immediately form.  You must use this within about 30-45 minutes.  The microscopic droplets of the resin-in-suspension will soak into the large, medium and small stucco/concrete pores, but will not get into the really microscopic porosity that concrete usually develops.  As you apply it to the bare stucco, you will see it soak into the porosity of the bare stucco at the edges of the crack, and as well any nearby surface that was cleaned of paint in order to do this repair.  If a little gets on the adjacent paint (usually latex) don’t worry;  It can be immediately wiped up with paper towels, and a little bit of the oily film left will stick to the old latex paint and you can paint new latex paint over it when you finish the repair.

Start in the morning of a day.

Apply the Damp Concrete Primer with a very narrow roller or (preferably) a brush.

Hold the roller only horizontally and roll only upwards.  The liquid will wet the surface and flow into the open porosity of the stucco.  The really important thing is that it wets the exposed stucco surface of the walls of the vee-groove in the stucco where you widened the crack.

When brushing, hold the handle well-above the bristle-pack and brush upwards, to wet all the stucco in the vee-groove.  If you brush with the bristles above the handle, the Damp Concrete Primer will run out of the bristle-pack, down the handle, down your wrist, perhaps past the elbow and possibly make it into your armpit.  You really don’t want to let any of that that happen.

The water-component of the DCP will evaporate over the next hour or so, and the surface will have a clear brownish oily film.  Over the next few hours it will go from slippery-wet to sticky-wet to dry-to-the-touch but still soft-to-the-fingernail. When it is no-longer-sticky, that’s the time to apply the caulk: in the afternoon of the same day.

Areas primed with the Damp Concrete Primer but not-caulked-that-afternoon will need to be primed again the morning of the next work-day.  If you don’t reprime, most caulks will not adhere all-that-well.  We want chemical bonds, to have a result that really lasts.

Since caulks are pastes or gels, they don’t really WET the surface unless you smear them against the surface;  feeding a bead out of a caulking-gun and laying-the-bead in the groove does not get actual wetting and thus not real adhesion, and this is easily a reason for a third if not half of all caulk-failures.  The stuff needs to be actually smeared against the surface.

The way to apply caulk is to fill the groove about a third-full to half-full, and then with a suitable tool shaped somewhat like the groove (or even a gloved finger-tip), run down the groove so as to smear the caulk against the groove sidewalls.  THEN you can fill the groove with caulk and squeegee flush.

Now, there’s one final step to have an invisible stucco-repair.  You must do this while the caulk is still sticky.  For latex-acrylic caulk that might be a very few hours.  For 3M5200 FAST, that might be about thirty to sixty minutes.  For 3M5200, that will be a day or two.

The last step is to press into the wet caulk some coarse sand or fine gravel:  Not to fill-the-crack, but only enough that a layer of the sand/gravel adheres in one surface layer on the sticky-caulk surface.  Now go away and leave it until the caulk has cured.  When you paint the area, the paint will drape over the sand/gravel stuck to the caulk surface, and it will have the same texture as the surrounding stucco, and no one passing-by will know there was ever a repair, if you match the shade of paint.

Please do not get chemical products on your bare skin, and do not to breathe the smell of any chemical product.  Have a fan at your back if in a confined area.  Use these or any chemical products with adequate ventilation, and use gloves when working.  Even if the labels on the containers don’t say, many resins are chemical irritants at best, meaning are hazardous if you get them on your skin a lot (or sometimes for some people, only a little).  They are quite safe when used wisely, in accordance with proper safety procedures and following the information on the container labels

----Steve Smith


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